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UC-NRLF 


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GIFT  OF 


UNIVERSITY  OF  CALIFORNIA 
DEPARTMENT  OF  MINING  AND  METALLURGY 

ASSAYING  LABORATORY 


S.  B.  CHRISTY 
Dean 


ABORATORY  INSTRUCTIONS  FOR 

FIRE  ASSAYS  OF  GOLD, 

SILVER,  AND  LEAD 

MINING  107  a-b 


BY 


WALTER  S.  MORLEY 
Assistant  Professor  of  Metallurgy 


UNIVERSITY  OF  CALIFORNIA 
DEPARTMENT  OF  MINING  AND  METALLURGY 

ASSAYING  LABORATORY 


S.  B.  CHRISTY 
Dean 


LABORATORY  INSTRUCTIONS  FOR 
!        FIRE  ASSAYS  OF  GOLD, 
SILVER,  AND  LEAD 

MINING  107  a-b 


BY 

WALTER  S.  MORLEY 
Assistant  Professor  of  Metallurgy 


UNIVERSITY  OF  CALIFORNIA   PRESS 

BERKELEY 

1913 


PREFACE 

These  instruction  sheets  have  been  prepared  in  this  form  to 
take  the  place  of  mimeograph  copies  which  were  formerly  used 
by  students  electing  the  course  in  fire  assaying  at  the  University 
of  California.  These  notes  are  concise  directions  for  the  carry- 
ing out  of  certain  laboratory  work,  and  in  no  wise  cover  the 
ground  of  the  lectures  which  supplement  the  instruction  in  the 
laboratory. 

The  purpose  of  the  course  is  not  to  teach  merely  practical 
fire  assaying,  but  also  to  familiarize  the  student  with  metal- 
lurgical principles  which  underlie  smelting  operations  on  the 
large  scale;  and  in  the  lectures  as  much  emphasis  is  placed  on 
the  underlying  metallurgical  principles  as  on  the  practical 
details  of  fire  assaying. 

The  laboratory  work  is  divided  into  twenty-five  assignments, 
thirteen  of  which  are  intended  to  cover  the  work  of  the  first 
half-year,  and  the  remaining  twelve,  the  work  of  the  second  half- 
year.  In  addition  to  the  twelve  assignments  of  the  second  half- 
year,  there  will  be  given,  as  time  permits,  assignments  relating 
to  the  fire  assays  of  mercury,  tin,  cobalt,  and  antimony,  and  the 
determination  of  carbon,  sulphur,  and  ash  in  coal  and  coke. 


323637 


TABLE  OF  CONTENTS 

CHAPTER  I 

PRELIMINARY  WORK  PAGE 

1.  Weighing  8 

2.  Practice  in  Cupellation  10 

3.  Parting  Gold  and  Silver  13 

4.  Fire  Assay  of  Lead  Bullion  14 

CHAPTER  H 

SCORIFICATION    ASSAYS 

5.  Scorification  in  General  15 

6.  Scorification  of  Silver  Ores  17 

7.  Quartz-Gold  Ores  containing  less  than  one-half  Pyrite  19 

8.  Quartz-Gold  Ores  containing  more  than  one-half  Pyrite  20 

9.  Scorification  Assay  of  Samples  High  in  Copper 21 

10.  Scorification  Assay  of  Ores  and  Products  Fusible  with  Difficulty.  21 

11.  General  Practice  22 

CHAPTER  III 

CRUCIBLE  ASSAYS 

12.  Determination  of  Gold  and  Silver  in  Litharge:   of  the  Reducing 

Powers  of  Charcoal,  Argol,  and  Flour,  and  Assay  of  Fluxes 23 

13.  Quartz  Ores   25 

14.  Nitre  Assay  for  Ores  containing  Moderate  Amounts  of  Pyrite 27 

15.  Nitre  Assay  for  Ores  containing  Large  Amounts  of  Pyrite 29 

16.  Roasting  Method  for  Ores  containing  Large  Amounts  of  Pyrite  ....  30 

17.  Assay  of  Sulphide  Ores  by  "Nail  Method"  31 

18.  Assay  of  Tailings  33 

CHAPTER  IV 

BULLION  ASSAYS 

19.  Assay  of  Gold  Bullion  35 

20.  Fire  Assay  of  Silver  Bullion  39 

CHAPTER  V 

MISCELLANEOUS  WORK 

21.  Furnace  Repairs.    Making  Cupels  40 

22.  Melting,  Refining,  and  Sampling  of  Gold  and  Silver  Bullion 41 

23.  Comparison  of  Methods  for  Assay  of  Copper-bearing  Materials  ....  42 

24.  Practice  in  Sampling,  Panning,  and  Amalgamating  43 

25.  Fire  Assay  of  Lead  Ores  45 

[5] 


THE  NOTE-BOOK 

Systematic,  neat  and  accurate  note-taking  is  of  just  as  much,  if  not 
more,  importance  to  the  student  of  fire  assaying  than  the  actual  details 
of  the  work  itself.  Careful  note-taking  involves  close  observation  and 
concentration.  A  mere  numerical  result,  without  a  statement  of  the 
conditions  governing  the  experiment,  is  not  wanted.  With  a  particular 
numerical  result  should  be  given  all  the  essential  facts  concerning  the 
experiment,  as  determined  by  observation,  reading  and  thought. 

System  and  neatness  are  essential  characteristics  of  a  good  engineer, 
especially  a  mining  engineer,  whose  duties  often  take  him  into  places 
where  hardship,  dirt,  and  disorder  are  the  rule. 

You  should  take  your  note-book  not  only  into  the  lecture  room,  but 
into  the  furnace  room  as  well.  Write  down  your  observations  while 
they  are  fresh  in  your  memory.  Keep  your  note-book  clean,  your  writing 
legible,  and  your  drawings  neat. 

Properly  taken  carbon  copies  are  expected,  and  they  should  be  handed 
in  not  later  than  one  week  after  the  experiment  has  been  performed. 


[6] 


REGULATIONS   COVERING  WORK  IN  THE   ASSAYING 
LABORATORY 

Before  commencing  work,  each  student  should  be  supplied 
with  the  folowing  articles : 

1.  Copy  of  Fulton's  Manual  of  Fire  Assaying. 

2.  Capsule-box   containing  six  porcelain   capsules,   three 
unglazed  capsules,  three  Kennedy  parting-flasks,  three 
test  tubes,  needle  and  magnifier. 

3.  Laboratory    folder    with    centimeter-ruled    paper,    ar- 

ranged for  carbon  copy. 

4.  HHH  lead  pencil. 

5.  Set  of  Becker  (or  equivalent)  weights  from  1  gram  to 

%0  milligram. 

6.  Pair  of  bone-tipped  pincers. 

7.  Pair  of  steel  pincers. 

8.  Pair  of  bead  pliers. 

9.  Blow-pipe. 

10.  Laboratory  suit. 

Each  student  should  report  promptly  for  work  at  the  ap- 
pointed hour,  and  should  stay  the  full  laboratory  period,  unless 
excused. 

All  fires  should  be  drawn  by  4:15  o'clock. 

All  apparatus  should  be  left  in  its  proper  place  and  in  order. 


[7] 


CHAPTER  I 

PRELIMINARY  WORK 

ASSIGNMENT  1 

WEIGHING 

In  this  assignment  you  are  expected  to  become  thoroughly 
familiar  with  the  construction,  adjustment,  use,  and  care  of  an 
assay  balance.  Read  carefully  all  the  references  given  at  the 
end  of  this  assignment. 

An  assay  balance  is  similar  to  a  chemical  balance,  except  that 
the  moving  parts  are  made  lighter,  and  instead  of  limiting  the 
sensibility  to  .10  mg.  it  is  made  to  weigh  accurately  to  .01  mg. 
With  the  balance  you  use  this  is  done  entirely  by  the  method  of 
deflections.  Later  on  you  will  be  given  a  balance  with  a  rider. 

Calculate  the  sensibility  of  the  balance  assigned  to  use,  as 
follows :  After  a  couple  of  oscillations  take  readings,  first  on  one 
side  and  then  on  the  other — for  example: 

PANS  EMPTY 

Left  Right 

—2.9  2.6 

—2.5  2.2 

—2.0  1.8 

—1.6 


4)     9.0  3)   6.6 


—2.25  2.2 

—2.25 
2.20 
2)~705~ 

—  .03      Resting  point,  left 

Repeat  the  readings  until  uniform  results  are  obtained.  If 
the  resting  points  do  not  agree,  increase  the  number  of  readings 
on  a  side.  After  a  little  practice,  two  readings  on  one  side  and 
one  on  the  other  will  suffice. 

In  this  assignment  record  all  readings  in  your  notes.  After 
you  have  become  skillful  in  reading,  recording  will  not  be  neces- 
sary. In  taking  readings  do  not  let  the  needle  vibrate  more  than 
a  total  of  five  divisions. 

[8] 


Now  place  your  !/<>  mg.  weight  on  the  left  pan,  allow  needle 
to  swing  as  before,  and  determine  the  new  resting-point — for 
example : 

14  MG.  WEIGHT  ON  LEFT  PAN 

Left  Right 

3.1  7.3 

3.3  7.0 

3.6 


3)   10.0  2)   ]4.3 

3.33  7.15 

7.15 
3.33 


2)   10.48 

5.24  2nd  resting  point,  right 

—    -03  1st  resting  point,  left 

5.27  Difference 


The  difference  between  the  two  resting  points  is  the  number 
of  divisions  on  the  ivory  index  through  which  y2  mo-  nas  caused 
the  needle  to  be  deflected. 

The  sensibility  of  a  balance  may  be  defined  as  the  deflection 
caused  by  1  mg. ;  or,  expressed  in  another  way,  the  sensibility 
is  the  weight  in  hundredths  of  a  milligram  which  will  cause  the 
needle  to  be  deflected  one  division  on  the  index. 

In  the  above  example,  the  sensibility  is  10.54  divisions,  or 
.095  mg. 

As  you  can  readily  read  to  tenths  of  a  division,  the  balance 
will  be  sensible  to  .01  mg.  Were  the  beam  perfectly  rigid,  this 
sensibility  would  hold  for  much  greater  loads  than  are  usually 
weighed  on  an  assay  balance ;  but  for  a  load  of  even  !/2  grm. 
there  is  a  perceptible  change.  To  test  this,  place  a  load  of  1  gram 
on  each  pan  of  the  balance  and  determine  the  sensibility  as  you 
did  when  the  load  was  zero.  Compare  results. 

You  are  now  ready  to  weigh  by  the  deflection  method.  Obtain 
a  number  of  pieces  of  gold  or  silver  weighing  from  1  to  10  mg. 
Weigh  each  separately  and  then  all  together.  Add  up  the 
separate  weights  and  compare  with  the  total  weight.  Repeat 
until  your  results  are  consistent. 

[9] 


Make  a  full  and  complete  record  of  your  observations. 

Before  leaving  the  balance  see  that  the  beam  is  lowered  and 
the  case  closed. 

The  foregoing  method  of  weighing  is  specially  adapted  to 
the  poorer  grades  of  balances  in  use.  When  a  rider  attachment 
is  available,  the  ''rider-deflection"  method  may  be  used,  the 
rider  taking  the  place  of  the  2  mg.  and  smaller  weights.  This 
method  is  the  one  ordinarily  used  in  assay  work.  With  an 
exceedingly  sensible  and  well-constructed  balance  the  method 
of  ''no  deflections"  (see  Fulton's  Manual  of  Assaying)  may  be 
used. 

Make  a  drawing  of  your  balance  shelf,  showing  the  method 
used  to  secure  a  firm  foundation :  give  dimensions  of  foundation 
supports,  width  of  shelf,  etc. 

REFERENCES 

Fulton:  Manual  of  Assayng,  pp.  42-52. 
Aaron:  Assaying,  part  I,  pp.  27-38  and  127. 
Thorpe:  Quantitative  Chemical  Analysis,  pp.  3-33. 

Catalogues   descriptive   of  the  balances  made  by   Becker,   Troemner, 
Ainsworth,  Smith  &  Thompson,  and  Keller. 


ASSIGNMENT  2 

PRACTICE  IN  CUPELLATION 

A.  Place  half  a  dozen  dry  cupels  in  the  muffle  and  build  a 
fire  in  the  usual  manner.  As  coke  is  used  for  fuel,  it  is  necessary 
to  have  a  good  coal  fire  started  before  adding  the  coke.  Let  the 
muffle  heat  up  until  a  bright  red  is  reached,  then  arrange  the 
dampers  so  that  the  heat  remains  uniform.  Before  cupelling, 
see  that  the  fire  is  solid  around  the  muffle. 

Now  obtain  six  lead  buttons,  weighing  approximately  15 
grams  each.  Cupel  in  sets  of  two  each  in  the  following  manner : 
Bring  two  of  the  cupels  forward  say  within  two  inches  of  the 
front  of  the  muffle.  Place  a  lead  button  in  each,  using  the  cupel 

[10] 


tongs,  then  shove  the  cupels  back  into  the  hotter  part  of  the 
muffle.  Close  the  muffle  door  for  a  few  minutes.  When  the 
buttons  have  melted  and  cupellation  has  commenced,  as  shown 
by  the  fumes  of  litharge,  open  the  door  and  arrange  the  dampers 
so  that  the  fumes  are  drawn  slowly  off  through  the  rear  of  the 
muffle.  Try  to  keep  the  temperature  constant,  increasing  it  only 
when  the  lead  has  almost  disappeared.  The  heat  is  too  low 
when  the  fumes  become  heavy  and  dark,  and  a  scum  forms 
around  the  edge  of  the  molten  lead.  The  heat  is  too  high  when 
the  fumes  of  litharge  rise  rapidly  to  the  top  of  the  muffle  and 
the  lead  appears  to  be  boiling.  The  heat  is  just  right  when 
crystals  of  litharge  form  in  the  cupel.  Practice  will  enable  you 
to  fix  upon  the  proper  conditions. 

Cupel  the  remaining  four  and  as  many  more  as  you  have  time 
for  during  the  period. 

Make  a  full  and  complete  record  of  your  observations. 

Study  carefully  the  following  references. 


REFERENCES 

Brown:  Manual  of  Assaying,  pp.  214-223. 
Furman:  Assaying,  pp.  138-142. 
Bodeman  &  Kerl:  Assaying,  pp.  110-124. 
Knead  &  Sexton:  pp.  109-115. 


B.  Roll  out  a  silver  coin  to  the  thickness  of  an  ordinary 
visiting  card.  Anneal  the  coin  repeatedly  during  the  operation. 
Clean  with  sandpaper.  Now  weigh  out  on  your  bead  balance  six 
assays  of  this  coin,  three  weighing  25-30  mgs.  each,  and  three 
weighing  90-100  mgs.  each.  Weigh  each  bead  accurately.  Wrap 
each  assay  in  15  grams  of  sheet  lead,  weighed  approximately  on 
the  pulp  balance.  Arrange  these  six  assays  in  sets  of  two  each, 
so  that  each  set  will  contain  one  25-30  mg.  assay  and  one  90- 
100  mg.  assay.  Cupel  one  set  at  a  time,  as  in  "A,"  only  using 
more  care  on  account  of  the  resulting  silver  bead.  Try  to 
feather  the  cupels  and  at  the  same  time  brighten  the  beads. 

[11] 


Remove  the  beads  from  the  cupels  and  clean  thoroughly  by 
brushing.  Place  them  in  their  respective  capsules,  and  take  to' 
the  balance  room.  Weigh,  and  report  in  this  form : 

Silver 
Date          Assay  No.         Coin  Lead          Wt.  Bead        fineness  Remarks 

Sept.  1  1  27.6  15  24.3  880         Well  feathered; 

brightened  at 
proper  heat. 

The  term  silver  fineness,  as  used  above,  refers  to  the  number 
of  parts  of  silver  found  in  1000  parts  of  the  coin. 

Compare  the  results,  and  note  the  average  percentage  silver 
losses  for  the  25-30  mg.  assays  and  for  the  90-100  mg.  assays, 
taking  into  account,  of  course,  the  known  fineness  of  the  silver 
coin  used. 

Dissolve  a  piece  of  coin  silver  in  nitric  acid  and  test  with 
ammonium  hydroxid  for  copper;  also  test  a  well-brightened  bead 
in  the  same  manner.  What  deductions  do  you  make? 

C.  Obtain  from  the  storeroom,  six  lead  buttons  or  packets, 
having,  respectively,  the  following  compositions: 

1.  Lead  and  silver. 

2.  Lead,  silver  and  gold. 

3.  Lead,  silver  and  antimony. 

4.  Lead,  silver  and  copper. 

5.  Lead,  silver  and  tin. 

6.  Lead,  silver  and  iron. 

Cupel  in  sets  of  two  each.  Endeavor,  in  each  case,  to  make 
a  perfect  cupellation.  Observe,  carefully,  in  each  case,  the  heat, 
time  and  manner  of  brightening;  observe  the  formation  of  any 
scoria,  its  color,  etc. ;  note,  in  each  case,  the  color  and  general 
appearance  of  the  cupel  after  brightening. 

Make  a  full  and  complete  report. 


[12] 


ASSIGNMENT  3 

PARTING  GOLD  AND  SILVER 

To  be  a  successful  assayer  one  must  be  able  to  part  correctly 
and  rapidly.  Nicety  of  manipulation  is  required,  and  the  only 
•  way  to  acquire  this  is  to  practice  conscientiously.  Do  not  take 
anything  for  granted.  There  are  three  steps  in  an  assayer 's 
work  where  he  must  be  sure,  viz:  (1)  That  the  sample  is  what 
it  represents;  (2)  that  nothing  but  gold  is  weighed  as  such; 
(3)  that  the  weighing  is  done  accurately.  With  these  points 
looked  to  carefully,  the  rest  will  be  comparatively  easy.  Perform 
carefully  the  following  experiments : 

Weight  out  six  pieces  of  gold  foil  on  your  bead  balance  to 
.01  mg. ;  each  piece  should  contain  about  5  mgs.  Alloy  the  pieces 
of  gold  with  1,  2,  3,  4,  5,  and  8  times  their  weight  of  silver, 
respectively.  Weigh  the  silver  also  on  your  bead  balance,  to 
.10  mg.  only. 

To  alloy,  wrap  the  gold  and  silver  in  about  1  gram  of  sheet 
lead  and  cupel  in  the  muffle,  taking  care  to  have  the  cupel 
smooth,  dry  and  hot.  The  muffle  should  be  slightly  hotter  than 
for  ordinary  cupellation.  After  alloying  remove  the  bead  from 
the  cupel  by  means  of  the  bead  plyers,  brush  thoroughly,  and 
place  in  the  porcelain  capsule  used  for  parting.  Treat  each  of 
the  five  beads  thus  and  remove  to  the  parting  room.  Now  flatten 
each  bead,  using  the  small  blowpipe,  hammer  and  anvil.  Return 
them  to  their  respective  capsules,  which  should  then  be  filled 
with  hot,  weak  parting  acid;  heat  slowly  on  the  hot  plate,  and 
when  all  action  has-  ceased,  replace  the  spent  acid  with  strong 
acid  and  heat  for  three  minutes  at  boiling  point.  Then  remove 
from  the  plate,  pour  off  the  acid,  and  wash  three  times  with 
distilled  water  (preferably  warm).  Dry  over  the  hot  plate,  then 
anneal  by  heating  to  redness  over  the  Bunsen  flame.  Cool  and 
weigh.  Report  results  in  this  form : 

Wt.  Gold  taken  Silver  Ratio          Wt.  of  Parted  Gold  Remarks 

6.21  1  to  1  12.10  Not  acted  upon  by  acid. 

What  do  your  experiments  determine? 

[13] 


Describe  minutely  the  action  of  the  acid  on  the  several  beads. 
NOTE. — Flatten   all   dore   beads   unless   they   are   known   to 
contain  more  than  eight  parts  of  silver. 


REFERENCES 

Brown:  Manual  of  Assaying,  pp.  224-239. 
Furman:  Manual  of  Assaying,  pp.  145-146. 
Bodeman  &  Kerl:  Assaying,  pp.  198-200. 
Fulton:  Manual  of  Assaying,  pp.  107-110. 


ASSIGNMENT  4 

FIRE  ASSAY  OF  LEAD  BULLION 

Chip  or  saw  off  four  y2  A.T.  samples  from  the  bullion  given 
you,  and  assay  according  to  the  directions  given  in  Fulton's 
Manual  of  Assaying. 

Report  both  gold  and  silver  values  in  this  form: 

Per  ton 


No.  of  Gold  Silver  Gold  Silver 

Date  bullion  Dore  nigs.  ings.  ozs.  ozs. 

Sept.  28  22  60.22  5.10  55.12  10.20  110.24 


To  get  correct  results  in  this  assignment  the  cupellation  must 
be  done  perfectly,  so  in  a  measure  this  assignment  will  test  your 
ability  to  cupel. 

Explain  the  term  A.T. 

REFERENCES 
Furman:  Manual  of  Assaying,  pp.  14-17  and  272-275. 


[14] 


CHAPTER  II 

SCORIFICATION   ASSAYS 
ASSIGNMENT  5 

SCORIFICATION  IN  GENERAL 

In  this  assignment  you  are  expected  to  get  a  general  idea  of 
the  scorification  process  as  far  as  can  be  done  by  reading. 
A  brief  outline  of  the  work  in  scorification  is  given  below : 

1.  Sorification  assay  of  silver  ores. 

(a)   Silver-lead  ores. 

(6)   Those  containing  no  lead,  but  quartz,  metallic  silver, 
and  silver  compounds. 

A  general  charge  for  ores  of  the  above  class  is : 

Ore  %0  A.T. 

Granulated  lead     30-35  grams 
Borax  glass  %  gram 

Use  2%  in.  scorifiers. 

This  charge,  with  slight  modifications,  will  treat  any  silver  ore; 
if  the  ore  contains  more  than  50  per  cent  of  pyrite,  the  lead 
may  be  increased  up  to  45  grams.  And  if  much  lime,  zinc  or 
antimony  is  present,  the  borax  glass  must  be  increased  up  to  as 
much  as  three  grams  in  exceptional  cases.  Where  the  sample 
is  rich  and  at  the  same  time  homogeneous  the  weight  of  the 
sample  taken  may  be  decreased  to  %0  A.T. 

2.  Scorification  assay  of  rich  gold  ores. 

As  will  be  noticed  from  your  reading,  the  "scorification 
assay"  is  primarily  adapted  for  silver  ores  and  rich  gold  ores, 
for  evident  reasons.  To  get  the  required  amount  of  sample  for 
a  medium  or  poor  gold  ore,  it  becomes  necessary  to  make  several 
scorifications  and  then  scorify  the  resulting  buttons  together, 
or  cupel  separately  and  part  together. 

[15] 


(a)  Quartz  gold  ores  consisting  largely  of  quartz.  —  The 
charge  given  for  silver  ores  can  be  used  equally  well  for  quartz 
gold  ores  containing  less  than  50  per  cent  pyrite,  though  in  most 
cases  it  is  better  to  take  more  than  */w  A.T.  and  consequently 
more  lead  and  a  larger  scorifier  if  necessary. 

When  yw  A.T.  of  ore  is  used  30  grams  are  sufficient,  and 
with  %  A.T.  50  grams  of  lead  had  better  be  used. 

(6)  Quartz-pyrite  gold  ores  consisting  largely  of  pyrite.  — 
Owing  to  the  large  amount  of  lead  required  to  scorify  ores  of 
this  class,  it  is  impracticable  to  use  more  than  l/w  A.T.  of  ore 
in  one  scorifier,  so  to  accomplish  the  same  result  it  becomes 
necessary  to  unite  the  lead  buttons  as  suggested  above. 

The  usual  charge  is  : 


Ore  Ko  A-T. 

Granulated  lead     40-45  grams 
Borax  glass  %  gram. 

3.  Scorification  assay  of  samples  containing  a  large  percentage 
of  copper. 

In  samples  of  this  class  it  is  necessary  to  use  a  small  amount 
of  ore  and  at  the  same  time  a  large  amount  of  lead  ;  this  is  due 
to  the  retention  of  copper  in  the  lead  and  the  difficulty  in 
removing  it  unless  a  sufficient  amount  of  lead  is  used.  If  any 
great  amount  of  copper  is  retained  in  the  lead  button,  a  high 
heat  will  be  necessary  to  remove  it  during  cupellation.  This 
will  produce  a  high  loss  in  silver  value  ;  so  it  is  almost  imperative 
to  add  enough  lead  during  scorification  to  insure  the  removal 
of  the  copper,  when  the  silver  loss  will  be  small  in  comparison. 

The  usual  charge  is  : 

Ore  i/10  or  i/20  A.T. 

Granulated  lead     45  or  50  grams 
Silica  and  borax. 

4.  Scorification    assay    of    ores    and    products    fusible    with 
difficulty. 

[16] 


Ores  containing  tin,  zinc  or  antimony  will  come  under  this 
head.  A  large  amount  of  borax  glass  and  a  high  heat  are 
necessary. 

REFERENCES 

Rhead  &  Sexton:  Manual  of  Assaying,  pp.  106-121. 
Furman:  Manual  of  Assaying,  pp.  122-126. 
Bodeman  &  Kerl:  Assaying,  pp.  107-116. 
Brown:  Assaying,  pp.  195-214. 


ASSIGNMENT  6 

SCORIFICATION  ASSAY  OF  SILVER  ORES 
Preparation  of  the  charge : 

On  all  assays  the  following  method  of  selecting  the  portion 
of  ore  to  be  assayed  (assuming  that  the  sample  is  correct  and 
ground  to  pass  a  100-mesh,  or  finer,  sieve)  should  be  rigidly 
followed.  If  there  be  any  doubt  as  to  the  uniformity  of  the 
sample,  pour  out  on  a  piece  of  glazed  paper  and  remix,  then 
replace  the  sample  in  the  bottle  or  bag.  Assay  charges  are  then 
to  be  taken  direct  from  the  bottle  or  bag  by  means  of  a  spatula. 

Consult  your  lecture  notes  for  an  explanation  of  this  method 
of  selecting  the  portion  of  ore  to  be  assayed. 

Copper  spoons  are  provided  for  measuring  the  granulated 
lead. 

Measure  one-half  the  lead  required  into  the  scorifier,  add  the 
ore,  mix  with  the  spatula,  spread  the  remaining  half  of  the  lead 
over  the  charge,  and  sprinkle  with  borax  glass  over  all. 

Sample A  silver-lead  ore. 

Prepare  six  charges  as  follows : 

Ore  i/10  A.T. 

Granulated  lead     30-35  grams 
Borax  glass  %  gram 

Use  2l/2  in.  scorifiers. 
[17] 


Make  the  six  scorifications  at  one  time  in  the  muffle  assigned 
to  you.  Have  the  muffle  at  a  bright  red  before  placing  the 
charges  in  it;  when  the  charges  are  in,  close  the  muffle  door.  In 
a  few  minutes  draw  the  door  aside  slightly,  and  if  the  lead  is 
melted,  remove  the  door  completely  and  arrange  the  drafts  so 
that  there  will  be  a  strong  current  of  air  through  the  muffle. 
During  the  scorification  the  heat  should  be  maintained  at  a 
medium  orange,  or  a  little  above  a  good  heat  for  cupelling. 
Increase  toward  the  end  to  a  full  yellow.  As  soon  as  the  '  *  bull 's 
eye"  has  disappeared,  pour  into  a  warmed  scorification  mould. 
When  all  the  charges  have  been  removed  from  the  muffle,  place 
six  cupels  in  so  they  may  be  ignited  by  the  time  the  buttons  are 
slagged  and  ready  for  cupelling. 

Cupel  the  six  buttons  at  one  time,  feathering  them  all.  Weigh 
each  bead  separately ;  part  the  beads  together  in  sets  of  three  in 
Kennedy  parting  flasks,  and  weigh  the  gold. 

Sample A  silver  ore  containing  a  quartzose  gangue. 

Sample  A  quartz-silver  ore. 

Prepare  six  charges  of  this  sample,  each  as  follows : 

Ore  %0  A.T. 

Granulated  lead     30-35  grams 
Borax  glass  %  gram 

Use  2y2  in.  scorifiers. 

Cupel  the  six  buttons  at  one  time,  feathering  them  all.  Weigh 
each  bead  separately ;  part  the  beads  together  in  sets  of  three  in 
test  tubes  and  weigh  the  gold.  Record  the  results  in  this  form : 

Per  ton 


Assay     Sample  Wt.  Wt.  Wt.  Oz.  Oz. 

Date         No.          No.  Ore  Dore  Gold  Gold        Silver 

Oct.  1         1         226         3/10A.T.         10.23mgs.         .04  mgs.  .4         101.9 

Average  the  results  of  the  assays  of  each  sample  after  calcu- 
lating them  separately  for  each  assay. 

Under  remarks  give  composition  of  the  ore,  manner  of 
scorifying  and  cupelling. 

If  there  is  any  reason  to  doubt  the  accuracy  of  the  assay  the 
results  should  never,  under  any  circumstances,  be  accepted. 

[18] 


Describe  in  your  notes  the  appearance  of  the  scorifiers  and  the 
character  of  the  lead  buttons.  Weigh  out  %0  A.T.  of  the  ore 
and  wash  on  a  batea,  making  yourself  familiar  with  the  con- 
stituent minerals  and  the  approximate  percentage  of  each.  Use 
the  blowpipe  if  necessary.  Make  a  detailed  report  in  your  notes. 

Follow  the  above  directions  in  all  future  assignments. 


ASSIGNMENT  7 

QUARTZ-GOLD   ORES   CONTAINING  LESS   THAN 
ONE-HALF   PYRITE 

Sample  

Make  three  assays  of  this  sample  as  follows : 

Ore  y10  A.T. 

Granulated  lead   30-35  grams 
Borax  glass  %  gram 

Use  2^/2  in-  scorifiers. 
and  three  assays : 

Ore  Vs  A.T. 

Granulated  lead     45-50  grams 
Borax  glass  %  gram 

Use  2%  in.  scorifiers. 

Unite  the  three  buttons  from  the  first  charge  by  re-scorifica- 
tion  before  cupelling.  Cupel  the  other  three  buttons  directly 
and  in  separate  cupels;  weigh  separately  for  dore;  part  one 
alone  and  two  together.  If  necessary  to  add  silver  before  part- 
ing, alloy  by  means  of  blowpipe  and  dry  cupel. 

Note  the  time  required  by  each  charge  and  the  size  of  the 
lead  button.  Reduce  by  scorifying,  the  size  of  any  button 
weighing  over  20  grams.  Observe  the  following  periods  in 
scorification  of  sulphide  ores  (consult  your  lecture  notes)  : 

1.  Melting.  Have  muffle  at  a  low  yellow  heat.  Introduce  the 
scorifier  and  close  the  muffle  door. 

[19] 


2.  Roasting.    When  the  charges  have  thoroughly  melted,  open 
the  door  carefully  and  allow  as  much  air  as  possible  to  pass 
through  the  muffle ;  if  they  begin  to  bubble,  close  the  door  so  as 
to  decrease  the  air  supply.     The  heat  during  the  roasting  period 
should  be  kept  at  about  a  medium  orange.     A  high  heat  will 
cause  mechanical  loss  by  too  rapid  oxidation. 

3.  Fusion,   or   formation   of   the   "bull's   eye."     When   the 
roasting  has  ceased,  close  the  door  and  fuse  down  the  chargn 
thoroughly. 

4.  Scorification.     When  the  charge  has  thoroughly  fused,  as 
shown  by  the  formation  of  the  "bull's  eye,"  open  the  door  and 
allow  a  good  current  of  air  to  pass  through  the  muffle.    During 
this  period  the  lead  content  is  reduced  and  the  slag  becomes  more 
fluid. 

5.  Final  heating.    When  the  lead  is  covered  or  almost  covered 
by  the  slag,  close  the  door  and  increase  the  heat  to  a  full  yellow, 
and  then  pour  into  heated  moulds. 


ASSIGNMENT  8 

QUARTZ-GOLD  ORES  CONTAINING  MORE  THAN 
ONE-HALF   PYRITE 

Sample  

Prepare  six  charges  of  this  sample,  as  follows : 

Ore  i/10  A.T. 

Granulated  lead     45-50  grams 
Borax  glass  }£  gram 

Use  2%  in.  scorifiers. 

Scorify  as  directed  in  previous  assignment;  cupel  separately, 
feathering  carefully.  Clean  the  beads  carefully,  combine  in  sets 
of  three,  weigh  and  part;  weigh  again.  Report  both  gold  and 
silver. 

[20] 


ASSIGNMENT  9 

SCORIFICATION  ASSAY  OF  SAMPLES  HIGH  IN  COPPER 

Sample  and  Sample  A  copper  matte. 

Prepare  five  charges  of  each  sample,  as  follows : 

Ore  i/10  A.T. 

Granulated  lead     50-55  grams 
Borax  glass  1  gram 

Use  3  in.  scorifiers. 

For  copper  matte  add  1  gram  of  silica  with  borax  glass. 
Control  the  furnace  as  in  assaying  ores  rich  in  pyrite.  Buttons 
from  matte  must  be  rescorified ;  use  2%  in.  scorifiers  and  have 
total  lead  40  grams.  Cupel  separately  the  five,  buttons  from 
each  sample,  feathering  the  cupels  at  first  and  raising  the  heat 
at  the  end  so  as  to  ensure  the  removal  of  all  the  copper.  Beads 
from  each  sample  should  be  weighed  separately,  but  parted 
together. 

What  are  the  difficulties  encountered  in  assaying  ores  rich 
in  copper? 

REFERENCES 

Furman:  Manual  of  Assaying,  pp.  290-291. 
Peters:  Modern  Copper  Smelting,  pp.  67-71. 


ASSIGNMENT  10 

SCORIFICATION 'ASSAY   OF   ORES   AND   PRODUCTS 
FUSIBLE  WITH  DIFFICULTY 

Sample  Contains  much  antimony. 

Prepare  three  charges  as  follows: 

Ore  1/10  A.T. 

Granulated  lead     30-35  grams 
Borax  glass  3-4  grams 

Use  2%  in.  scorifiers. 
[21] 


Scorify  in  the  usual  manner  except  at  the  end,  when  the  heat 
must  be  raised  to  a  full  yellow.  The  buttons  should  be  perfectly 
malleable;  if  they  are  not,  rescorify  with  an  addition  of  test 
lead.  Cupel,  feathering  carefully;  weigh  the  beads  separately. 
Part  in  one  capsule. 


ASSIGNMENT  11 

GENERAL  PRACTICE 

Take  the  sample  given  you  and  reduce  by  methods  suggested 
until  you  have  at  least  two  pounds  passing  a  100  mesh  sieve ; 
bottle  this  and  keep  for  assay. 

In  this  assignment  you  will  be  given  three  or  more  ores,  in 
addition  to  the  one  sampled.  You  are  to  determine  their  com- 
position and  then  decide  on  charge  you  would  use  for  assay  by 
scorification  method. 

Make  a  detailed  report,  indicating  the  composition  of  each 
sample,  the  charge  used,  etc.  Give  both  gold  and  silver  values. 


[22] 


CHAPTER  III 

CRUCIBLE  ASSAYS 

ASSIGNMENT  12 

DETERMINATION  OF  GOLD  AND  SILVER  IN  LITHARGE  : 

OF  THE  REDUCING  POWERS  OF  CHARCOAL, 

ARGOL,   AND   FLOUR,   AND   ASSAY 

OF  FLUXES 

Before  making  any  assays,  the  assayer  should  determine  the 
precious  metal  content  of  the  litharge  used.  Litharge  contain- 
ing more  than  a  trace  of  gold  should  be  rejected.  The  presence 
of  a  small  but  uniform  amount  of  silver  is  rather  an  advantage 
than  otherwise,  as  the  silver  serves  to  help  collect  the  gold. 

The  assayer  must  also  determine  the  reducing  powers  of  the 
various  reducing  agents;  and  an  assay  of  all  the  reagents  used 
should  be  made  in  order  to  detect  any  "salting"  of  the  fluxes. 

First.  To  determine  the  precious  metal  content  of  the 
litharge,  prepare  two  charges  as  follows : 

Litharge  100  grams 

Argol  2  grams 

Salt  cover. 

Use  a  No.  7  crucible. 

To  one  charge,  add  a  small  amount  of  test  silver.  Fuse 
quickly  both  charges  in  a  hot  fire  until  thoroughly  liquid,  then 
withdraw  and  pour;  cupel  the  resulting  buttons.  Determine 
gold  and  silver  by  weighing  the  unalloyed  bead  for  the  dore, 
and  parting  the  other  for  gold.  If  more  than  a  trace  of  gold  is 
found,  the.  litharge  must  be  rejected.  Note  the  silver  content, 
and  correct  all  assays  according  to  the  amount  of  litharge  used. 

For  example : 

Silver  in  100  grams  of  litharge  weighs  .50  mg.,  litharge  used 
in  assays  weighs  60  grams,  dore  bead  weighs  2.24  mg.,  then 
.60  X  -50  =  .30  mg.,  and  the  corrected  dore  is  2.24  — .30,  or 

1.94mg. 

[23] 


Second.  To  determine  the  reducing  powers  of  the  reducing 
agents,  argol,  flour  and  charcoal,  prepare  charges  as  follows: 

1.  Argol  1  gram 
Silica  30  grams 
Litharge  75  grams 
Soda  25  grams 
Borax  cover. 

Use  a  No.  F  crucible. 

2.  Substitute  for  argol, 

Flour  1  gram 

3.  Substitute  for  argol, 

Charcoal  1  gram 

Reducing  agents,  only,  should  be  weighed  carefully.  Fuse, 
at  one  time,  the  above  charges ;  when  quiet,  pour  into  clean,  warm 
slag  moulds.  Clean  and  weigh  the  resulting  lead  buttons. 
Weigh,  using  pulp  scales,  to  the  nearest  10  mg. 

Note  that  the  above  determinations  are  made  under  normal 
working  conditions. 

Third.  To  make  an  assay  of  the  fluxes  used,  prepare  one 
charge,  as  follows: 

Silica  30  grams 

Litharge  75  grams 

Soda  25  grams 

Borax  glass  5  grams 

Reducer  for  18-gram  button. 
Salt  and  Borax  cover. 

Use  a  No.  F  crucible. 

Add  a  weighed  amount  of  test  silver  to  the  charge,  and  fuse 
in  the  usual  way  Determine  gold  and  silver,  if  any. 


[24] 


ASSIGNMENT  13 

QUARTZ  ORES 

In  ores  of  this  class  are  included  only  those  whose  principal 
constituent  is  quartz,  with  the  addition  of  0—15  per  cent  of  iron 
pyrites.  Any  exception  will  be  noted  below.  The  following 
charge  will  treat  any  ore  of  this  class : 

Ore  1  A.T. 

Litharge  75  grams 

Soda  25  grams 

Borax  glass  5  grams 

Salt,  borax  glass,  or  ground  borax  cover. 

Reducer,  preferably  flour,  to  reduce  18-22  gram  button. 

Observe  first,  that  1  A.T.  of  ore  is  used.  This  is  the  standard 
amount  for  ordinary  crucible  assays.  When  the  ore  is  rich  in 
gold  it  is  advisable  to  take  less,  say  %  or  !/4  A.T. ;  when  the  ore 
is  poor  in  gold,  as  in  stamp  mill  tailings,  it  may  be  advisable  to 
take  as  much  as  4  A.T. 

The  amount  of  litharge  in  a  charge  for  ore  of  this  character 
should  be  such  that  every  particle  of  the  ore  is  oxidized.  For 
an  ore  containing  minimum  amount  of  pyrites,  60  grams  of 
litharge  will  suffice.  Any  great  excess  is  to  be  guarded  against, 
as  the  crucible  is  apt  to  be  corroded  and  the  assay  lost. 

The  third  element  of  the  charge  is  soda  (sodium  carbonate)  ; 
this  flux  is  used  in  every  crucible  assay.  To  some  extent  it  may 
replace  the  litharge;  so  that  if  the  soda  in  the  above  charge  is 
increased,  the  litharge  can  be  decreased,  thereby  making  a  more 
economical  charge,  although  the  slag  would  not  be  so  clear. 

Borax  as  borax  glass  is  a  most  useful  flux.  If  the  ore  to  be 
treated  contains,  besides  the  quartz,  any  great  amount  of  infusible 
oxides,  the  borax  may  be  increased  up  to  20  grams. 

A  cover  of  salt  or  borax  (hydrous)  should  be  used  on  all 
assays ;  it  acts  as  a  wash  and  prevents  an  overflow. 

[25] 


Sufficient  reducer  should  be  used  to  produce  an  18-25  gram 
lead  button.  If  the  ore  is  oxidizing,  several  grams  of  reducer 
may  be  necessary.  If  the  ore  contains  more  than  about  10  per 
cent  pyrites  and  no  oxidizing  agent,  reducer  will  be  at  a  mini- 
mum and  a  small  amount  of  oxidizer  as  nitre  may  be  necessary. 
An  ore  containing  more  than  about  15  per  cent  pyrites  cannot 
be  properly  treated  by  this  charge. 

Other  reducing  agents  which  may  be  used  instead  of  flour 
are  argol,  charcoal  or  sugar. 

Prepare  three  charges  of  Sample  ,  using  the  charge 

given  before  and  an  F  crucible.  This  sample  consists  mainly  of 
quartz  with  less  than  5  per  cent  of  sulphides  of  iron,  lead  and 
copper,  and  reduces grams  of  lead  per  one  assay  ton  of  ore. 

Unless  the  proportion  of  silver  to  gold  in  an  ore  is  known  to 
be  at  least  five  to  one,  test  silver  should  be  added  to  all  charges 
except  one.  Weigh  the  test  silver  added  to  one  of  the  charges. 

This  illustrates  three  methods  of  working:,  and  we  have  two 
checks  on  the  gold  and  two  on  the  silver.  The  amount  of  silver 
in  the  litharge  will  of  course  be  deducted  from  the  weight  of 
the  dore. 

On  one  charge  use  a  salt  cover,  on  the  others  a  borax  cover. 

Heat  the  charge  until  in  quiet  fusion,  then  increase  the  heat 
to  low  yellow  for  about  five  minutes,  or  until  the  charge  becomes 
smooth  on  top.  Pour  into  clean,  warm  slag  moulds,  and  when 
cool  break  away  the  lead  button;  clean,  hammer  into  cubes,  and 
then  cupel  in  the  usual  manner. 

Report  results  in  the  following  form : 

Per  ton 

A 

Gold 

Assay     Sample  Gold  Silver       Gold          Silver       @  $20.67 

No.          No.  Ore  Dore  mg.  mg.  oz.  oz.  per  oz. 

1         483         1A.T.         2.22         1.00         1.22         1.00         1.22  $20.67 

Under  remarks  indicate  everything  that  has  a  bearing  upon 
the  results.  Describe  the  appearance  of  slag,  buttons,  etc.  Which 
cover  do  you  prefer,  and  why  ? 

[26] 


Make  a  vanning  test  of  the  ore,  using  about  %0  A.T.  of  ore 
and  a  batea.  Eeport  this  test  also. 

If  necessary  to  use  %  A.T.  of  ore  or  to  make  the  fusions  in  a 
muffle,  the  following  charge  will  be  found  suitable : 

Ore  y2  A.T. 

Litharge  60  grams 

Soda  20  grams 

Borax  glass  5  grams 

Eeducer  to  give  a  20-gram  button. 
Ground  borax  cover. 

Use  a  No.  F  crucible. 

Prepare  two  charges  of  Sample  ,  using  the  above 

charge.  Add  a  weighed  amount  of  test  silver  to  one  charge,  and 
an  unweighed  amount  of  test  silver  to  the  other.  Make  the  usual 
tests,  and  fuse  as  before. 

To  mix  any  charge,  put  the  soda  into  the  crucible  first,  then 
the  litharge  and  other  fluxes,  lastly  the  ore,  and  mix  all  together 
with  a  fork;  sprinkle  salt  or  borax  over  the  top.  Make  certain 
the  ore  and  fluxes  are  thoroughly  mixed. 


ASSIGNMENT  14 

NITRE  ASSAY  FOR  ORES  CONTAINING  MODERATE 
AMOUNTS  OF  PYRITE 

Ores  of  this  class  are  usually  richer  in  gold  and  silver  than 
those  of  the  previous  class,  and  consequently  less  material  may 
be  taken  in  assaying  them. 

The  following  charge  will  be  found  suitable  for  ores  of  this 
class : 

Ore  1/2  A.T. 

Litharge  90  grams 

Soda  20  grams 

Silica  10  grams 

Borax  glass  5  grams 

Nitre  according  to  preliminary. 

Cover. 

Use  a  No.  7  fluxing  pot. 
[27] 


Fusion  of  the  charges  should  be  carried  on  as  before. 

Calculate  the  amount  of  nitre  (see  below)  to  add  so  that  a 
20-gram  button  will  be  produced  from  %  A.T.  of  ore. 

Make  three  asays  of  Sample ,  using  the  charge  given 

above.  Sample contains  about sulphides. 

Add  test  silver  to  all  charges  but  one.  Before  commencing 
work  on  this  assignment,  be  sure  that  the  previous  one  is 
thoroughly  understood. 

The  reducing  agents  which  are  commonly  present  in  ores  are 
antimony,  zinc,  arsenic  and  sulphur.  Iron  pyrites,  the  sulphide 
commonly  met  with,  reduces  as  follows: 

Fifteen  per  cent  of  pyrite  will  reduce  about  nineteen  grams 
of  lead,  and  there  will  be  no  excess.  More  than  15  per  cent  of 
pyrite,  using  %  A.T.  of  ore,  will  reduce  too  large  a  lead  button. 
Nitre,  an  oxidizer,  is  used  to  control  the  size  of  the  lead  button, 
and  is  usually  stated  in  terms  of  lead ;  that  is,  one  gram  of  nitre 
oxidizes  four  grams  of  lead,  more  or  less,  according  to  conditions. 

In  order  to  know  how  much  nitre  to  add,  so  as  to  reduce, 
within  a  few  grams,  a  20-gram  lead  button,  we  must  determine 
or  guess  correctly  the  amount  of  lead  which  will  be  reduced 
before  making  an  assay. 

The  reducing  power  of  a  sample  may  be  determined  by  making 
a  preliminary,  thus: 

Sample  i£0  A.T. 

Litharge  75  grams 

Salt  cover. 

Use  a  No.  7  fluxing  pot. 

Have  the  fire  hot  when  the  charge  is  put  in,  and  fuse  quickly ; 
when  in  quiet  fusion,  remove,  and  pour.  Clean  and  weigh  the 
button. 

Calculate  the  amount  of  nitre  which  must  be  used  on  a  basis 
of  one  gram  of  nitre  to  four  grams  of  lead. 

In  preparing  charge  of  Sample  ,  add  to  one  charge 

i/2  gram  more  nitre  and  to  the  other  y2  gram  less  nitre  than  the 
calculated  amount.  Weigh  the  resulting  lead  buttons,  and 
calculate  the  oxidizing  power  of  nitre  under  these  conditions. 
Compare  this  result  with  the  amount  used  above. 

[28] 


ASSIGNMENT  15 

NITRE   ASSAY  FOR   ORES   CONTAINING   LARGE 
AMOUNTS  OF  PYRITE 

Material  of  composition  as  indicated  above,  is  in  general, 
assayed  in  the  same  way  as  that  of  the  previous  assignment. 
The  increase  in  the  amount  of  iron  pyrites  calls  for  an  increase 
of  litharge  to  complete  the  oxidation ;  and  as  the  amount  of 
silica  in  the  ore  is  decreased  and  the  charge  becomes  more  basic, 
more  acid  in  the  form  of  silica  or  window-glass  must  be  added. 

The  following  charge  will  suffice : 


Ore 

1/2  A.T. 

Litharge 

120  grams 

Soda 

20  grams 

Silica 

15  grams 

Borax  glass 

10  grams 

Nitre  according  to  preliminary. 
Salt  or  Borax  cover. 

Use  a  No.  F  or  30-gram  crucible. 


Calculate  for  20  gram  button. 

When  the  material  to  be  assayed  is  a  pure  concentrate  or 
nearly  so,  the  amount  of  nitre  to  be  added  can  usually  be  placed 
at  about  25  grams  for  y2  A.T.  of  ore.  For  material  of  unknown 
percentage  of  pyrites,  a  preliminary  assay  must  be  made. 

In  general,  ores  of  this  type  are  richer  than  the  type  treated 
in  Assignment  14,  and  a  smaller  amount  of  ore  can  be  assayed, 
thus  effecting  a  saving  in  time  and  fluxes;  when  a  sensitive 
balance  is  available  and  the  values  are  uniformly  distributed, 
the  results  obtained  by  using  a  smaller  amount,  14  A.T.,  will 
be  even  more  accurate  than  those  from  %  A.T. 

[29] 


The  following  charge  will  do  when  14=  A.T.  is  used : 


Ore 

%A.T. 

Litharge 

75  grams 

Soda 

15  grams 

Silica 

5  grams 

Borax  glass 

5  grams 

Nitre  according  to  preliminary. 
Salt  or  Borax  cover. 

Use  a  20-gram  crucible. 

Calculate  for  16-20  gram  button. 

The  ore  to  be  assayed  in  this  assignment  is  a  concentrate 
obtained  from  one  of  our  mill  runs,  and  is  a  typical  concentrate. 

Prepare  three  charges  of  Sample ,  using  y2  A.T.,  and 

three  using  14  A.T. 

Time  required  for  fusion  about  thirty  minutes,  ten  minutes 
yellow  heat  at  the  end.  Avoid  heating  too  rapidly,  as  a  sudden, 
uneven  heating  may  cause  the  assay  to  boil  over. 

Report  the  mean  of  the  several  assays  as  the  correct  value, 
first  discarding  any  assay  that  appears  to  have  been  abnormal 
in  any  manner. 


ASSIGNMENT  16 

ROASTING  METHOD  FOR  ORES  CONTAINING  LARGE 
AMOUNTS  OF  PYRITE 

Obtain  two  4-inch  roasting  dishes  with  covers.  Line  each 
dish  thoroughly  by  rubbing  a  blackboard  crayon  over  its  interior. 

Weigh  out  %  A.T.  of  Sample into  each  dish.  Place 

the  dishes,  covered,  in  the  muffle,  which  should  be  heated  to  low 
redness  only  around  the  edges  and  corners.  The  material  to  be 
roasted  should  be  spread  out  in  the  roasting  dishes  and  ridged 
across,  so  as  to  expose  as  much  surface  as  possible  to  the  air. 
After  one-half  hour,  take  out  the  roasts,  letting  them  cool  a 
little  before  removing  the  covers;  then  turn  the  ore  over  with 
a  wire  provided  for  the  purpose,  leaving  it  spread  about  and 

[30] 


ridged,  as  before.  Return  the  dishes,  without  covers,  to  the 
muffle,  if  the  heat  is  still  low.  After  one-half  hour  take  out  and 
stir  again.  Continue  this  treatment,  gradually  increasing  the 
heat,  till  no  odor  of  sulphur  dioxide  is  perceived  on  stirring. 
Finally,  increase  the  heat  to  a  bright  red  for  about  ten  minutes, 
to  decompose  any  iron  sulphate  which  may  have  formed. 
Smelt  the  roasted  concentrate  with  this  charge : 

Concentrates  roasted 

Litharge  60  grams 

Soda  15  grams 
Flour  4  grams 

Silica  15  grams 

Borax  glass  10  grams 
Borax  cover. 

Use  a  No.  F  crucible. 

After  the  concentrates  have  been  transferred  to  the  crucible, 
the  roasting  dish  should  be  ''dry-washed"  out  with  a  little  soda, 
which  is  added  to  the  charge. 

Compare  results  with  those  obtained  from  the  nitre  assay. 


ASSIGNMENT  17 

ASSAY  OF  SULPHIDE  ORES  BY  THE  "NAIL  METHOD" 

The  "nail  method"  is  adapted  to  gold  ores  containing  vary- 
ing amounts  of  sulphides,  although  it  may  with  certain  modifi- 
cations be  applied  to  all  gold  ores.  Ores  containing  appreciable 
amounts  of  arsenic  and  antimony  cannot  be  assayed  by  this 
method,  with  this  exception :  arsenic  frequently  occurs  in  vanner 
concentrates  and  may  then  be  removed  in  the  first  stages  of  the 
assay  by  oxidation  with  nitre,  part  of  the  sulphur  being 
oxidized  at  the  same  time. 

In  general,  gold  values  will  be  higher  by  this  method  and 
silver  values  lower. 

[31] 


Prepare  two  charges  of  Sample as  follows : 

Ore  i/2  A.T. 

Litharge  30  grams 

Flour  2.5  grams 

Soda  45  grams 

Salt  or  borax  cover. 
Three  twenty-penny  nails. 

Use  a  No.  F  crucible. 


Add  a  weighed  amount  of  test  silver  to  each  charge. 

Stick  the  nails  into  the  charge  after  the  borax  cover  has  been 
added,  points  down.  Should  the  nails  first  added  be  eaten  up 
by  the  charge  before  fusion  is  finished,  take  out  what  remains 
of  the  old  ones  and  add  two  or  three  fresh  ones. 

First  fuse  the  charges  in  the  usual  way;  when  foaming  has 
ceased,  give  a  very  bright  yellow  heat  for  ten  or  fifteen  minutes, 
continuing  the  heating  until,  on  removing  the  nails  and  rinsing 
them  in  the  slag,  they  are  seen  to  be  free  from  adhering  globules 
of  lead. 

Prepare  one  charge  of  Sample as  directed  for  Sample 

,  except  add  5  grams  S1O2,  and  add  no  test  silver. 

Prepare  two  charges  of  Sample as  follows: 


Ore 

1/2  A.T. 

Litharge^ 

25  grams 

Soda 

45  grams 

Borax  glass 

10  grams 

Silica 

15  grams 

Five  twenty-penny  nails. 
Salt  or  borax  cover. 

Use  a  No.  F  crucible. 


Make  the  fusion  as  instructed  before. 

When  arsenic  is  present  it  is  well  to  omit  the  nails,  add  10 
to  20  grams  of  nitre,  and  * '  boil  down ' '  the  charge ;  then  add  the 
nails  and  finish  at  a  high  heat  as  usual. 

[32] 


In  each  of  the  above  cases  compare  results  with  those  obtained 
from  the  litharge  assay. 

What  do  your  results  indicate? 

In  what  way  is  the  nail  assay  convenient  ? 


ASSIGNMENT  18 

ASSAY  OF  TAILINGS 

Mine  samples  usually  consist  of  one  or  all  of  the  following: 
quartz,  quartz  and  varying  amounts  of  sulphides,  concentrates 
and  tailings.  You  have  had  practice  in  assaying  all  the  above 
except  the  last.  Tailings  are  composed  (i.e.,  stamp-mill  tailings) 
mainly  of  quartz.  No  sulphide  should  be  present. 

It  is  often  necessary  to  determine  values  in  tailings  as  close 
as  five  cents  per  ton.  This  would  require  4-5  A.T.  to  be  taken 
at  one  time,  or  a  number  of  smaller  charges,  and  the  resulting 
buttons  combined.  In  the  latter  case  5-8  gram  buttons  could  be 
reduced  and  cupellation  effected  without  previous  scorification. 

The  following  charges  can  be  used  to  advantage: 


Ore 

Litharge 

Sodium  carbonate 
Flour 

Borax  glass 
Borax  cover. 
Crucible 


2  A.T. 

3  A.T. 

4  A.T. 

120  grams 

180  grams 

240  grams 

60  grams 

90  grams 

100-120  grams 

2  grams 

2  grams 

2  grams 

10  grams 

10  grams 

10  grams 

Sample Prepare  three  charges,  using  2  A.T.,  3  A.T. 

and  4  A.T.    Add  one  or  two  mgs.  of  silver  to  each  charge. 
Report  in  the  usual  way. 


[33] 


CHAPTER  IV 

BULLION  ASSAYS 

Bullion,  in  general,  is  classified*  as  follows: 

1.  Lead  bullion,  usually  the  product  of  the  lead  blast-furnace ; 
95  per  cent  and  more  lead,  containing  some  copper,  antimony, 
etc.,  silver  and  gold. 

2.  Base  bullion,  containing  from  100  to  925  parts  of  silver 
per  1000,  gold  in  varying  amounts,  and  a  large  percentage  of 
base   metals,    chiefly    copper,   zinc,    lead,    etc.      Produced   most 
frequently  by  cyanide  mills. 

3.  Dore  bullion,  containing  925  to  990  parts  of  silver  per 
1000,  some  gold,  and  base  metals,  mostly  copper,  but  also  lead, 
antimony,  zinc,  etc. 

4.  Fine  silver  bullion,  free  from  gold,  containing  990  to  1000 
parts  silver  per  1000,  but  some  base  metals,  usually  copper. 

5.  Silver  bullion,  containing  little  base  metal  and  less  than 
half  its  weight  in  gold. 

6.  Gold  bullion,  containing  little  base  metal  and  more  than 
half  its  weight  in  gold. 

7.  Fine  gold  bullion,  free  from  silver,  containing  from  990  to 
1000  parts  gold  per  1000. 


You  have  had  practice  in  assaying  lead  bullion  (see  Assign- 
ment 4).  Gold  bullion  and  fine  gold  bullion  may  be  considered 
under  a  general  case;  likewise  base  bullion,  dore  bullion,  fine 
silver  bullion,  and  silver  bullion. 


*  Fulton:  Manual  of  Assaying. 


[34] 


ASSIGNMENT  19 

ASSAY  OF  GOLD  BULLION 

The  bullion  that  you  are  to  assay  in  this  assignment  is  made 
to  represent  as  closely  as  possible  that  which  comes  from  Cali- 
fornia mines. 

The  Preliminary. — In  this  class  of  bullion  an  expert  can  tell 
with  sufficient  exactness  the  fineness  of  a  particular  sample  and 
so  can  alloy  in  the  proportion  of  2%  to  1,  without  making  a 
preliminary  survey.  Ordinarily  the  base  metal  fineness  is  not 
taken  into  account,  a  small  amount  of  copper  being  added  to  the 
proof  center  to  prevent  sprouting,  and  to  toughen  the  bead. 
But,  for  the  general  case,  and  for  completeness,  a  preliminary 
assay  must  be  made  in  order  to  determine  the  approximate  fine- 
ness of  gold,  silver  and  base. 

Weigh  out  90-100  mgs.  of  the  bullion  given  you ;  also  300  mgs. 
of  test  silver;  wrap  silver  and  bullion  together  in  3  grams  of 
sheet  lead,  and  cupel  at  a  little  above  the  feathering  point. 
Brighten  well;  clean,  and  part  the  bead. 

Report  the  results  according  to  the  following  example : 

Bullion  taken  100.0  mgs. 

Test  silver  300.0  mgs. 

Bullion  and  test  silver  400.0  mgs. 

Cupelled  bead  385.0  mgs. 

Fineness 

Base  metal  15.0  mgs.  150 

Gold  (by  parting)  80.0  mgs.  800 

Silver  (by  difference)  5.0  mgs.  50 


1000 


These  results  should  be  corrected  by  adding  15-20  points  to 
the  silver  fineness  and  substracting  the  same  amount  from  the 
base  metal  fineness.  This  is  to  allow  for  silver  lost  during 
cupellation. 

[35] 


Having  determined  the  approximate  fineness  of  the  bullion 
given  you,  weigh  out  two  samples  of  500  mgs.  each  (within 
.05mgs.);  also  weigh  out  test  silver  (within  .10  mgs.)  so  that 
the  ratio  of  silver  to  gold  will  be  2y2  to  3. 

Make  a  proof  center  of  pure  gold,  silver  and  copper  to 
correspond  with  the  regular  samples. 

Wrap  each  sample  in  6  grams  of  sheet  lead,  or  as  many  as 
the  following  table  calls  for.  For  bullion  containing  over  900 
points  of  precious  metal  5  grams  of  lead  are  sufficient  to  remove 
all  the  base  metal.  The  following  table*  is  valuable  for  determin- 
ing the  least  amount  of  lead  to  use : 

Precious  Metal  in  Alloy  Lead  required 
.900  10  parts 

.800  16  parts 

.700  22  parts 

.600  24  parts 

.500-.050  34  parts 

Cupellation. — Before  introducing  the  samples  into  the  muffle, 
have  the  temperature  at  a  bright  red  (higher  than  for  ordinary 
cupellation),  and  the  cupels,  which  should  be  smooth,  hot. 

Place  the  lead  packets  in  their  respective  cupels,  the  proof 
in  the  center,  and  then  close  the  muffle  door.  As  soon  as  they 
uncover,  open  the  door  and  allow  the  cupellation  to  proceed  in 
the  usual  manner. 

Take  care  that  no  metal  is  sticking  to  the  sides  of  the  cupels. 
Brighten  the  cupels  at  a  bright  red  heat,  cover  with  hot  cupels 
and  remove  gradually  from  the  muffle.  Clean  carefully  and 
weigh. 

Now  hammer  the  beads  into  the  shape  of  a  rectangular  bar, 
then  roll  out  to  the  thickness  of  an  ordinary  visiting  card  by 
means  of  the  rolls ;  or  in  the  absence  of  rolls,  hammering  will  do 
In  rolling,  anneal  the  alloy  whenever  it  becomes  hard  and  elastic 

*  Mitchell:  Manual  of  Assaying,  p.  610. 

[36] 


and  before  it  begins  to  get  rough  on  the  edges.  The  three  beads 
should  be  of  the  same  thickness  after  rolling.  Great  care  should 
be  taken  to  prevent  mechanical  loss. 

Parting. — Clean  thoroughly  three  matrass  flasks.  Place  2  ozs. 
of  1.16  sp.  gr.  nitric  acid  in  each;  now  introduce  the  flattened 
and  annealed  beads,  first  rolling  them  into  a  spiral  coil,  and 
heat  to  boiling. 

Boil  gently  until  action  ceases  (ten  to  fifteen  minutes),  then 
pour  off  the  spent  acid  and  add  2  ozs.  of  strong  acid  (1.26  sp. 
gr.).  Boil  gently  with  this  acid  for  fifteen  to  twenty  minutes. 
Place  a  piece  of  charcoal  or  burnt  brick  in  each  flask  to  prevent 
bumping.  Pour  off  this  acid  and  wash  the  gold  three  times  in 
hot  water.  Transfer  to  annealing  cups  and  anneal. 

Weigh  the  gold  carefully;  and  record  the  results  according 
to  the  following  example: 


Bullion 

Pure  gold 

Test  silver 

Copper 

Sheet  lead 

Total  gold  and  silver 

Cupelled  bead 

Cupellation  loss- 
Proof  center  loss 

Base  metal 
Parted  gold 
Pure  gold 

Surcharge 

Corrected  gold 

Base  metal 

Silver  (by  difference) 

Silver  fineness 
Base  metal  fineness 
Gold  fineness 


Sample  A 

Proof  center 

Sample  B 

500.0  mg. 

500.0  mg. 

400.0  mg. 

975.0  mg. 

1000.5  mg. 

975.6  mg. 

75  mg. 

6  grams 

6  grams 

6  grams 

1475.0  mg. 

1400.5  mg. 

1475.6  mg. 

1395.0  mg. 

1395.2  mg. 

1396.3  mg. 

80.0  mg. 

4.3  mg. 

79.3  mg. 

4.3  mg. 

4.3  mg. 

75.7  mg. 

75.0  mg. 

402.1  mg. 

400.5  mg. 

402.0  mg. 

400.0  mg. 

.5mg. 


.5mg. 


.5mg. 


401.6  mg. 
75.7  mg. 
22.7  mg. 

45.4 
151.4 

803.2 


401.5  mg. 
75.0  mg. 
23.5  mg. 

47.0 
150.0 
803.0 


[37] 


Remarks. — Gold  results  should  agree  within  14  point.  Silver 
results  will  depend  on  the  care  used  in  cupellation,  as  will  the 
base  metal  fineness.  Surcharge  is  the  difference  between  gold 
lost  during  cupellation  and  parting  (gold  may  be  lost  by 
volatilization,  cupel  absorption,  or  solution  in  the  acid)  and  the 
amount  of  silver  retained  in  the  parted.  It  is  generally  positive. 
A  negative  surcharge  is  to  be  regarded  with  suspicion. 

In  order  to  get  precise  results  it  is  necessary  to  have  pure 
gold  for  the  proof ;  parted  gold  from  previous  assays  will  do,  but 
then  an  arbitrary  surcharge  will  have  to  be  used,  say  i/4  to  1 
point. 

When  rolls  cannot  be  obtained,  the  gold  bullion  assay  may 
be  made  as  follows : 

1.  Make  a  preliminary  assay  as  usual. 

2.  Weigh  out  250  mgs.  of  bullion,  instead  of  500  mgs.,  and 
add  sufficient  test  silver  to  make  the  proportion  3  to  1.     Make 
up  the  proof  in  the  usual  way. 

After  weighing  the  cupelled  beads,  hammer  until  flat ;  anneal 
and  part,  using  acid  of  1.10  sp.  gr.  Boil  for  five  minutes,  decant, 
wash  three  times,  anneal  and  weigh. 

This  method  has  the  advantage  of  being  much  more  rapid 
than  the  former  and  at  the  same  time,  if  carefully  done,  is 
sufficiently  accurate. 


EEFEEENCES 

Eose:  Metallurgy  of  Gold,  pp.  462-492. 

Aaron:  Assaying,  part  2,  pp.  30-47. 

Pack:   "Assaying  of  Gold  and  Silver  in  the  U.  S.  Mint,"  Min.  and 

Sci.  Press,  Nov.  14,  1903. 
Whitehead  &  Ulke :  « '  The  Sampling  and  Assaying  of  Gold  Bullion, ' '  The 

Engineering  and  Mining  Journal,  Feb.  12,  1898. 

[38] 


ASSIGNMENT  20 


FIRE  ASSAY  OF  SILVER  BULLION 

Make  an  assay  of  the  sample  of  silver  bullion  obtained  in 
Assignment  22,  following  the  directions  given  in  Fulton 's  Manual 
of  Assaying. 

Record  results  according  to  the  following  example: 

PRELIMINARY  ASSAY 
Bullion  500  mg. 

Lead  10  grm. 

Cupelled  bead  440.4  mg. 

Parted  gold  none 

Silver  to  be  used  in  proof  445-450  mg. 

Gold  to  be  used  in  proof  none 

Copper  to  be  used  in  proof  50-55  mg. 


FINAL  ASSAY 


Bullion 

Silver 

Gold  in  proof 

Copper  in  proof 

Lead 

Gold  and  silver  in  proof 

Cupelled  bead 

Cupellation  loss 
Proof  center  loss 

Base  metal 
Parted  gold 

J3ase  metal  fineness 
Gold  fineness 
Silver  fineness 


Sample  A 
500  mg. 

Proof 
447.8  mg. 

Sample  B 
500  mg. 

none 

3.5  grm. 
442.5  mg. 

52  mg. 
3.5  grm. 
447.8  mg. 
444.8  mg. 

3.5  grm. 
443.0  mg. 

57.5  mg. 
3.0  mg. 

3.0  mg. 

57.0  mg. 
3.0  mg. 

54.5  mg. 

54.0  mg. 

none 

none 

none 

109 

108 

000 

000 

891 

892 

1000 


1000 

Use  the  utmost  care  in  cupelling.  Do  not  neglect  to  cover 
the  cupels  with  hot  cupels  when  the  beads  have  brightened;  a 
bead  that  has  sprouted  must  be  discarded  and  the  entire  assay 
repeated.  EEFERENCES 

Fulton:  Manual  of  Assaying,  pp.  175-177. 
Furman:  Manual  of  Assaying,  pp.  276-280. 

[39] 


CHAPTER  V 

MISCELLANEOUS  WORK 

ASSIGNMENT  21 

FURNACE  REPAIRS.     MAKING  CUPELS 

1.  You  are  assigned  to  a  particular  furnace,   and  you  are 
expected  to  keep  it  in  repair.    Examine  it  carefully ;  especially  see 
that  the  muffle  is  in  good  condition.     If  it  is  not,  remove  it  and 
put  in  a  new  one.     The  following  points  should  be  considered  in 
placing  a  new  muffle :  Have  the  muffle  well  supported  in  the  back 
and  front,  give  it  a  slight  inclination  downwards  toward  the 
front,  and  have  the  cementing  material  of  the  right  consistency. 
Fire-clay  with  a  certain  proportion  of  sand  or  broken  fire-brick 
is  the  cementing  material  used. 

Make  up  the  cement  in  the  following  manner :  Take  fire-clay 
and  fire-brick  in  the  proportion  of  one  to  three.  Crush  the  fire- 
brick to  pass  an  eight  or  ten  mesh  screen.  Mix  the  two  dry  and 
then  add  water  until  the  whole  is  in  a  plastic  condition.  Moisten 
thoroughly  the  brick-work  and  those  portions  of  the  muffle  that 
come  in  contact  with  the  clay,  then  press  the  clay  in  firmly  and 
allow  to  dry  at  least  twenty- four  hours. 

Make  a  drawing  (plan  and  sections)  of  your  furnace,  to  scale. 

2.  Make  200  good  cupels,  as  follows : 

Take  the  box  used  for  the  purpose  and  fill  with  bone-ash, 
add  water  and  mix.  Use  as  many  ounces  of  water  as  pounds  of 
bone-ash.  The  right  amount  of  water  has  been  added  when  the 
bone-ash  loses  its  harsh,  gritty  feeling,  and  at  the  same  time  is 
not  pasty.  A  small  amount  of  potassium  carbonate  (2  per  cent 
solution)  may  be  added  to  assist  in  cementing  the  bone-ash,  but 
this  is  not  necessary.  When  the  bone-ash  is  thoroughly  mixed 
and  is  of  the  right  consistency,  sift  through  a  twenty  mesh  sieve 
and  then  place  a  sufficient  amount  in  the  mould,  and  hammer 
down  firmly;  two  hard  blows  are  enough;  do  not  use  anything 
but  the  wooden  mallet  in  striking  the  mould.  Place  your  cupels 
on  a  board  and  set  away  to  dry. 

[40] 


ASSIGNMENT  22 

MELTING,  REFINING  AND   SAMPLING  OF  GOLD  AND 
SILVER  BULLION 

Melting. — The  object  of  this  experiment  is  to  make  you 
familiar  with  some  of  the  conditions  under  which  a  gold  bar 
of  moderate  size  is  melted  and  cast. 

Build  a  solid  fire  in  one  of  the  wind  furnaces.  Anneal  a 
No.  12  graphite  crucible,  if  one  has  not  already  been  annealed, 
and  place  in  the  fire.  Then  place  the  copper  bar  given  you  in 
the  crucible,  and  when  the  copper  is  melted  cover  with  powdered 
charcoal  to  prevent  oxidation.  When  the  metal  is  perfectly  fluid, 
pour  into  a  large  mould  that  has  been  first  heated  and  oiled. 
When  the  metal  is  solified,  turn  the  mould  over,  and  the  bar 
can  then  be  readily  removed.  Immerse  the  bar  in  a  dilute 
solution  of  sulphuric  acid,  and  scour  with  sand.  Weigh  in 
avoirdupois  pounds  and  ounces.  Convert  into  troy  ounces. 

Calculate  the  value  of  this  bar  on  the  supposition  that  it  is  a 
gold  bar  of  the  fineness  of  the  bullion  assayed  in  Assignment  19. 
Give  the  value  of  the  silver  in  troy  ounces  only,  of  the  gold  in 
dollars  as  well  as  ounces. 

Refining  and  Sampling. — Weigh  the  silver  bar  given  you, 
recording  the  result  in  troy  ounces. 

Anneal  (if  necessary)  a  graphite  crucible  of  proper  size,  and 
place  in  the  furnace.  Then  place  the  silver  bar  in  the  crucible. 
When  the  silver  is  thoroughly  melted,  refine,  following  instruc- 
tions as  given  in  references,  and  lecture  notes. 

W7hen  sufficiently  refined,  pour  or  cast  the  bar  as  directed  for 
the  copper  bar.  Clean,  dry  and  weigh,  as  before. 

Take  samples  by  cutting  off  from  diagonally  opposite  corners, 
two  or  three  grams  of  metal.  Keep  these  samples  separate. 
Assay  as  directed  in  Assignment  20.  Reweigh  the  bar.  Make 
detailed  report. 

EEFERENCES 

Aaron:  Assaying,  part  2,  pp.  21-46. 
Kose:  Metallurgy  of  Gold,  p.  387. 

[41] 


ASSIGNMENT  23 

COMPARISON  OF  METHODS  FOR  ASSAY  OF  COPPER 
ORES  AND  COPPER-BEARING  MATERIALS 

Scarification. — The  standard  charge  for  the  assay  of  copper 
ores  and  copper  bearing  material  by  the  scorification  method  is 
as  follows : 

Ore  Ho  A.T.  to  ifa  A.T. 

Granulated  lead  50  grams 
Borax  glass  1  gram 

Silica  1  gram 

Use  3  in.  scorifiers. 

Re-scorify  lead  buttons,  using  2%  inch  scorifiers  and  enough 
test  lead  to  make  total  weight  of  lead  not  less  than  35  grams. 
When  the  ore  or  copper-bearing  material  is  poor  in  copper  the 
silica  and  re-scorification  may  be  omitted.  As  many  assays  may 
be  run  as  are  necessary  to  give  accurate  results. 

Cupel  and  slag  corrections,  either  or  both,  are  sometimes 
required  and  may  be  made  as  follows: 

The  cupels  are  taken  in  lots  of  two  each,  the  unused  portion 
discarded,  the  remainder  crushed  to  pass  100  mesh,  and  fused 
with  100  grams  of  litharge,  20  grams  sodium  carbonate,  40  grams 
borax  glass,  and  3  grams  of  argol. 

The  slag  is  merely  ground  to  pass  100  mesh,  and  fused  with 
the  addition  of,  say,  40  grams  of  litharge,  20  grams  of  sodium 
carbonate  and  3  grams  of  argol. 

In  both  cases  the  resulting  lead  buttons  are  cupelled  and 
the  silver  and  gold  obtained  added  to  the  first  weights. 

Crucible. — A  standard  method  for  the  assay  of  the  above  by 
the  crucible  method  is  as  follows: 

Ore  Ho  to  %0  A-T. 

Litharge  80  grams 

Silica  12  grams 

Sod.  Garb.  16  grams 

Borax  glass  12  grams 

Eeducer  or  oxidizer  for  a  20  gram  button. 
Use  a  No.  F  crucible. 
[42] 


For  ores  or  other  material  containing  less  than  40  per  cent 
copper,  the  amount  used  for  assay  may  be  increased  up  to  %  A.T. 
Some  or  all  of  the  silica  may  be  omitted. 

Prepare  eight  charges  of  Sample ,  a  copper  matte 

rich  in  copper,  according  to  the  methods  already  outlined,  four 
by  scorification  and  four  by  crucible  fusion.  Use  proper  pre- 
cautions in  melting  and  cupellation. 

Make  a  tabular  report,  showing  comparative  losses,  etc. 


ASSIGNMENT  24 

PRACTICE  IN  SAMPLING,  PANNING  AND 
AMALGAMATING 

Sampling. — Take  the  sample  of  ore  given  you,  weigh,  and 
then  reduce  in  size  (weight  and  mesh)  by  approved  methods. 
Your  assay  sample  should  weigh  at  least  %  Ib.  and  should  pass 
at  least  a  100-mesh  screen.  Take  care  that  all  scales  pass  the 
screens  used. 

Reserve  all  discarded  ore  passing  the  40-mesh  screen  and  keep 
for  the  panning  and  amalgamating  experiments. 

Amalgamating :* — Weigh  out  five  charges  (100  grams  each) 
of  the  40-mesh  lot,  and  place  in  bottles  provided  for  the  purpose 
with  mercury  (10  grams),  and  150  c.c.  of  water;  and  agitate  for 
thirty  minutes.  Separate  the  mercury  by  panning  (see  below) 
and  transfer  to  a  porcelain  dish,  dry  with  filter  or  blotting  paper. 
Place  the  thoroughly  dry  mercury  in  a  20  gram  or  F  crucible 
in  which  20  grams  of  lead  have  been  placed.  Add  to  the  crucible 
a  charge  consisting  of  30  grams  of  litharge,  10  grams  of  soda, 
5  grams  of  borax  glass,  and  0.5  grams  of  argol,  and  silver  enough 
to  part  the  gold. 

Fuse  the  charge  very  slowly,  starting  with  a  slow  fire  in  the 
wind  furnace  or  if  using  a  muffle,  one  that  has  not  yet  become 


*  A  modification  of  Fulton 's  method. 

[43] 


red  and  has  a  good  draught  through  it  to  prevent  the  escape 
of  fumes  into  the  room.  When  fusion  is  quiet,  pour  and  cupel 
the  resulting  lead  button.  Part  and  weigh  the  gold. 

Panning. — The  contents  of  the  bottles  are  to  be  panned 
separately  and  then  together.  Pour  the  contents  of  one  bottle 
into  a  gold  pan,  add  water  if  necessary,  and  wash  by  giving  the 
pan  a  vibratory  motion  as  in  vanning,  washing  off  the  light 
particles  of  gangue  and  leaving  only  the  mercury  and  the  heavy 
portion  of  the  ore,  as  black  sand,  sulphides,  etc.  The  light 
portion  of  the  ore  should  be  washed  into  a  pan  or  vessel  of 
somewhat  larger  size  than  the  gold  pan.  Repeat  the  panning  of 
the  mercury  and  concentrates  in  order  to  separate  the  mercury. 
If  the  mercury  is  floured,  add  a  small  globule  of  liquid  sodium 
amalgam. 

Treat  the  contents  of  the  other  bottles  in  the  same  way.  The 
light  portion  or  tailings  of  the  several  bottles  may  be  poured 
into  the  same  vessel.  Re-pan  this  material  to  make  sure  that 
no  mercury  or  sulphides  have  escaped.  If  any  is  recovered,  add 
to  the  amount  recovered  before. 

Treat  the  collected  mercury  as  directed  above  (see  amalga- 
mating). 

Allow  the  tailings  to  settle,  pour  off  the  surplus  water,  and 
dry  by  any  convenient  means.  When  dry,  weigh,  pass  through 
a  100-mesh  screen,  mix  well  and  assay. 

Wash  the  heavy  material  or  concentrates  into  a  small  por- 
celain pan,  or  if  more  convenient  on  to  a  batea.  Examine  under 
a  magnifying  glass  or  low  power  microscope,  and  note  the 
different  kinds  of  sulphides  or  other  constituent,  and  the  approxi- 
mate amount  of  each. 

Dry,  weigh,  crush  to  pass  a  100-mesh  screen,  and  assay. 

Prom  your  results  calculate  the  following: 

1.  Assay  value  of  ore  in  ounces,  and  dollars  and  cents  per  ton. 

2.  Gold  amalgamated  in  ounces,  and  dollars  and  cents  per  ton. 

3.  Assay  value  of  concentrates  in  ounces,  and  dollars  and 
cents  per  ton. 

[44] 


4.  Assay  value  of  tailings  in  ounces,  and  dollars  and  cents  per 
ton. 

5.  Percentage  of  gold  amalgamated. 

6.  Total  gold  contents  in  comparison  with  assay  value. 

7.  Percentage  and  different  kinds  of  sulphides  or  other  heavy 
material. 

REFERENCES 

Fulton:  Manual  of  Assaying,  pp.  36-4  land  154-156. 

Furman:  Manual  of  Assaying,  chap.  2. 

Brown:  Manual  of  Assaying,  pp.  185-194. 

Hersam:    "Principles  in  the  Practice  of  Sampling,"  Min.  and  Eng. 

World,  Nov.  25,  1911. 
Brunton:   "Modern  Practice  in  Ore  Sampling,  Trans.  Am.  lust.  Min. 

Eng.,  vol.  40,  p.  567. 
Hofman:   The  Metallurgy  of  Lead,  chapter  V. 


ASSIGNMENT  25 

FIRE  ASSAY  OF  LEAD  ORES 

Lead  assays  are  usually  made  in  the  muffle ;  but,  in  the  absence 
of  such,  a  wind  furnace  may  be  used.  The  muffle  is  to  be 
preferred  because  the  heat  can  be  more  uniformly  controlled, 
and  there  is  less  danger  of  loss  by  volatilization. 

All  our  lead  assays  will  be  made  in  the  muffle  furnace. 

The  fire  assays  of  lead  ores  may  be  placed  under  three  heads, 
according  to  the  kind  and  proportion  of  flux  used : 

1.  Fusion  with  sodium  bicarbonate,  borax,  argol,  and  iron. 

2.  Fusion  with  mixture  of  sodium  bicarbonate  and  potassium 
carbonate,  borax,  argol,  and  iron. 

3.  Fusion  with  potassium  cyanide. 
Sample  Pure  assayers'  litharge. 

[45] 


Prepare  three  charges  as  follows: 

Ore  10  grams 

Lead  flux  35  grains 

Salt  cover. 

Use  Battersea  P  crucibles. 

Manage  heats  as  directed  below  j  pour  into  scorification 
moulds. 

Clean  buttons  by  brushing  and  hammering.  Weigh  on  pulp 
balance  to  .01  gram.  Report  to  .10  per  cent. 

Sample  Pure  Galena. 

Prepare  three  charges  as  before,  with  the  addition  of  four 
nails  to  each  crucible,  and  five  grams  of  borax  glass  on  top  of 
each  charge  before  salt  cover. 

Assay  Sample  in  the  same  manner. 

Your  results  should  check  to  within  .5  per  cent.  Write  your 
notes  carefully. 

The  lead  flux  is  a  mixture  of  sodium  bicarbonate  (12  parts) 
potassium  carbonate  (12  parts),  argol  (3  parts). 

The  first  method  should  be  used  only  when  potassium  car- 
bonate cannot  be  procured.  The  third  should  be  used  only  on 
pure  ores.  The  second  is  preferable  in  all  cases. 

Heats. — The  fusions  are  best  managed  by  giving  a  low  heat 
for  about  twenty  minutes  and  then  a  high  heat  to  finish.  This 
is  necessary  for  two  reasons :  ( 1 )  Lead  and  its  compounds  are 
volatile;  and  if  the  heat  is  so  high  that  boiling  takes  place 
violently,  some  lead  is  almost  certain  to  be  lost.  (2)  Lead  forms 
a  double  sulphide  with  sodium ;  and  to  reduce  this  by  means  of 
iron,  a  high  heat  is  necessary. 

The  nails  should  not  be  removed  until  entirely  free  from  lead. 

Fusion  with  cyanide  requires  a  very  low  heat. 

EEFEEENCES 

Hofman:  The  Metallurgy  of  Lead,  chapter  V. 
Fulton:  Manual  of  Assaying,  pp.  199-201. 

[46] 


— -saws.""" 

OVERDUE. 


lord  Bros. 

Makers 

Syracuse,  N.  Y. 
W.  JAN.  21,  1908 


YC   18949 


32368? 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


